This invention relates to cleaning and separation methods useful in cleaning substrates, particularly metal substrates, and useful for polymerization processes, coatings, extractions, and the manufacture and treatment of particles.
The cleaning of contaminants from workpieces is an important step in many manufacturing processes. Unfortunately, many processes employ environmentally undesirable solvents, or are high temperature processes that are energy intensive. For example, vapor degreasing techniques employ both volatile organic solvents and high temperatures. Efforts to replace such processes with aqueous systems are not entirely satisfactory because of the problem of contacting water to substrates that may be oxidized thereby, and by the problem of cleaning the contaminated water. In addition, the drying of aqueous systems is very energy intensive.
Vegetable oils such as soybean oil and modified soybean oil have been suggested for cleaning, but have not received significant use because of either their high cost or the difficulty in removing or extracting residual components of the oil.
CO2-based cleaning methods have been suggested. Some employ supercritical CO2, which (due to the need to handle higher temperatures and/or pressures) increases the cost of the apparatus used to carry out the processes. U.S. Pat. No. 5,377,705 to Smith et al. describes a precision cleaning system in which a variety of different co-solvents may be included (see column 8, lines 19-24 therein), with the mixture of the carbon dioxide and the co-solvent being either homogenous or heterogenous (see column 6, lines 4-11 therein). A problem with this system that it still does not provide a means to separate the contaminant from the co-solvent (see column 7, lines 24-32).
Accordingly, an object of the present invention is to provide a carbon-dioxide based cleaning system incorporating a separate cleaner, in which the contaminants may be separated from the cleaner to facilitate subsequent re-use or disposal of the cleaner.
A second object of the invention is to provide oil-based separation systems in which the oil, such as a vegetable oil, may be recovered for subsequent reuse.
A separation method comprises (a) providing a heterogeneous separation system, the heterogeneous cleaning system comprising CO2 in a first phase and an oil in a separate second phase; (b) entraining a material to be separated in the second phase; (c) solubilizing the second phase, in whole or in part, in the first phase to produce a separation system in which said material to be separated is insoluble; and then (d) separating the material from the system. The separating step is preferably followed by the step of (e) recovering the oil (i.e., some or all of the oil), so that it may be re-used in or recycled to step (a) above. Each of the steps may be carried out with or without agitation.
An advantage of the invention is that the separation system is phase-tunable, in that the material of the second phase can alternately be rendered soluble, in whole or in part, or insoluble in the first phase, alternately rendering the material to be separated soluble or insoluble in the system in a controllable manner. Thus the system is a homogeneous system in one embodiment, when the second phase is wholly solubilized in the first phase to render the material to be separated insoluble therein.
The system is useful in a variety of applications, including cleaning, polymerization, extraction, coating, and particle formation and treatment. The system is particularly advantageous where the oil employed is of a relatively high cost. Since environmentally acceptable solvents such as organic or vegetable oils (including synthetic oils) can be relatively expensive, this system enables the use of such products in a broader variety of applications, in a cost-effective manner.
As noted above, one particular aspect of the invention is a method of cleaning a contaminant from a substrate. The method comprises contacting a substrate with a heterogeneous cleaning system. The heterogeneous cleaning system comprising CO2 in a first phase and a cleaner (preferably an oil such as an organic, or vegetable, oil) in a separate second phase, so that contaminant carried by said substrate is entrained in the cleaner. The cleaner is then wholly or partially solubilized in the first phase (e.g., by increasing the pressure of the system) to produce a cleaning system in which the contaminant is immiscible (e.g., a homogeneous cleaning system), and that contaminant is separated from the substrate. The substrate is separated from the cleaning system, either before or after the solubilizing step, and the contaminant (which has been rendered immiscible in the cleaning system) is separated from the cleaner. The cleaning system advantageously can be implemented as a non-aqueous system, thereby reducing drying times and problems with oxidation.
The foregoing and other objects and aspects of the present invention are explained in detail below.